While it is clear that the secretions of the testis are adequate to inhibit gonadotropin secretions by the anterior pituitary gland, the contributions of the various circulating steroid (and non-steroid) compounds to this process are not clearly defined. We are investigating the possible role of two major metabolites of testosterone (dihyrotestosterone and estradiol) in this process and, as a necessary adjunct, are assessing the (unique?) ability of the rat to convert nearly all of the dihydrotestosterone which enters the blood stream to another adrogen (androstanediol). Through a series of experiments centering on orchidectomy and immediate steroid treatment we will attempt to relate the possible contributions of these metabolites to the inhibition normally exerted by the intact testis. Additional studies will center on the possibility of identifying steroid metabolites (from the liver) which contribute to this process.